Urea is commonly used as a fertiliser, supplying nitrogen to plants. Many soils also require sulphur as a plant nutrient, so fertilisers containing both urea and elemental sulphur have been developed. Desirably the elemental sulphur needs to be present as small dispersed particles to allow its oxidation in the soil to the plant available sulphate ion.
U.S. Pat. No. 3,100,698 discloses a urea-sulphur fertiliser that is made by combining molten urea and molten sulphur and subjecting the mixed melt to a prilling process. The mixed melt can also be prepared by adding solid urea prills to molten sulphur, or by adding solid sulphur to molten urea.
Melting sulphur and melting urea can be an energy intensive and therefore costly process, and can require sizeable equipment. Additionally, if melting of urea is not done quickly and in a controlled manner (i.e. the temperature is controlled such that it does not significantly exceed the melting point of urea), there is a risk of urea degradation. In particular, there is a risk of biuret production. Biuret is the condensation product of two molecules of urea and is a phytotoxin: a material that is toxic to plants. It can be formed when urea is heated or maintained at an elevated temperature for long periods of time. Biuret interferes with nitrogen metabolism and protein synthesis in plants. It is desirable, therefore, to minimise or at best eliminate the amount of biuret in compositions intended for use as fertilisers or soil improvers.
Further, when molten urea and molten sulphur are mixed, reactions may take place leading to the formation of impurities such as Thiourea. Thiourea is toxic for aquatic systems and a suspected carcinogen. Thiourea can be generated by the reaction of the dissolved hydrogen sulphide and polysulphides in sulphur and urea. The extent of the formation of these impurities is influenced by the time required to co-process molten urea and molten sulphur. Minimizing the mixing of molten urea and molten sulphur is hence key when processing these materials. The present inventors have sought to provide an improved process for the preparation of urea-sulphur fertiliser which is desirably simpler and more energy efficient than known processes. Preferably the process will allow for minimising the time at which urea is maintained at an elevated temperature and, thus, the reducing formation of biuret in the fertiliser compositions.